High and low pressure injection molding devices are well-known in the art for their use in producing plastic components. In conventional injection or structural foam molding machines, liquified plastic is produced by an extruder and transferred to one or more accumulators. The accumulators then force the molten plastic into a mold that includes a cavity designed to produce a selected part. Often, the mold is a “split mold” which includes two portions that separate to facilitate the removal of the formed part.
It is known in the prior art to couple multiple mold stations to a single extruder. To accommodate multiple molds, prior art designs make use of resin accumulators and injectors. For example, U.S. Pat. No. 6,241,508 B1 issued Jun. 5, 2001, to John et al., and commonly assigned with the present invention, discloses a multiple mold workstation with a single injection feeder and hydraulic pumping station. U.S. Pat. No. 6,241,508 B1 teaches the use of a resin accumulator and injection unit for each separate mold station. The resin accumulator employs a hydraulically driven piston that has a step-down reduction chamber to increase the pressure of the molten plastic as it is injected into the mold station.
Although widely used, accumulators of the type disclosed in U.S. Pat. No. 6,241,508 B1 increase the overall residence time of the molten plastic in the injection molding equipment. As such, additional energy must be consumed to maintain the molten state of the plastic during the extended residence time. Heater bands are commonly employed to maintain the temperature of the molten plastic between the extruder and the mold.
Another deficiency in the prior art is the limited versatility of conventional injection molding equipment. Prior art fixed-volume accumulators are configured to retain and deliver a quantity of molten plastic (“shot size”) based on the specific capacity of individual molds. Accordingly, it is difficult to change molds without also modifying the accumulators to accommodate different shot sizes. Too much plastic will make the mold “flash” open, and too little will create formation and ejection problems.
Yet another deficiency in the prior art is the inefficient method by which a split mold is held in a closed position during the injection process. Clamping pressure is required to oppose the internal pressures developed within the mold cavity. In the past, suitable clamping pressure is obtained through use of a hydraulically powered ram. Although effective, maintaining the clamping pressure through the continuous application of hydraulic pressure is energy intensive and inefficient.
In light of the foregoing deficiencies of the prior art, there is a need for more energy efficient injection molding equipment and methods.